Project Summary/Abstract Depression is the most common complication of pregnancy and affects 10-20% of women. Up to 75% of women with perinatal depression do not receive mental health care. In lower income African American and Hispanic women, the prevalence of perinatal depression (PND) is particularly high and the treatment rates are lower. Cognitive behavioral therapy (CBT) is an effective treatment but practical barriers such as time commitment, insurance coverage, and transportation, as well as stigma, limit uptake of CBT particularly for minority women. Similarly, concern about the adverse effects of antidepressant treatment on maternal and infant outcomes limits uptake of pharmacological treatments. A novel avenue for the development of safe therapeutics for depression focuses on the brain-gut axis and is supported by initial demonstrations that probiotics can improve mood symptoms. Our long-term goal is to assist in the discovery of potential microbes that are correlated with PND, and could be novel biomarkers for the diagnosis and treatment of this disorder, particularly in minority women. As a first step, we propose a multi-PI feasibility study involving the implementation and optimization of processes for participant recruitment and stool sample collection, as well as the initial assessments of longitudinal differences in gut microbiota between pregnant women with and without major depressive disorder (MDD). To meet these goals, we will leverage an established research infrastructure to conduct a prospective, nested case-control study. We will prospectively collect depression diagnoses and gut microbiome samples on 300 women at four points during pregnancy and after delivery to yield a total of 58 women, 29 cases with MDD and 29 controls without MDD. The two groups will be matched on key covariates (e.g., age, race, socio- economic status, gestational age, health). At their scheduled usual care, prenatal appointment, consented participants will complete mental health, demographics, microbiome-related, and dietary and physical habits questionnaires and provide a fecal sample either at that time of their appointment or at home. To achieve our first aim - to determine the feasibility of a larger similarly designed study - we will estimate the total number of potential participants, the number assessed and not assessed for eligibility (and reasons), the number excluded and reasons for exclusion, the number lost to follow-up, and the data available for analysis at each of the 4 visits. To achieve our second aim - to determine whether the gut microbiome dynamically differs in pregnant women with and without MDD - we will employ a suite of systems level approaches, including multivariate statistical techniques, network modeling and machine learning techniques. This R03 will allow us to optimize our research processes for the prospective collection of stool samples and mental health data in preparation for larger nested case-control studies, and ultimately clinical trials of probiotic therapies for PND.